One system is known in which graphics are formed by causing a layer of adhesive material to selectively adhere to a layer of granules by selectively heating the adhesive in graphic patterns so that upon separation of the layers, graphics will be formed by transfer of the granules to the adhesive layer. This system is described in U.S. Pat. No. 4,123,309, assigned to the assignee of this application.
Briefly, in that system graphics are formed along a composite strip material comprising (1) an accepting portion or tape comprising a layer of latent adhesive material and (2) a transfer portion or tape comprising a donor web carrying a lightly adhered layer of microgranules in face-to-face contact with the layer of adhesive material. At least one of the microgranule and adhesive layers bears a radiation absorbing pigment, and the strip material is essentially transparent to radiant energy between one exterior surface and the pigment so that the pigment may be exposed to heat-producing radiation. Upon momentary exposure to a pattern of radiation, the pigment is selectively heated and momentarily softens the adjacent portions of the layer of adhesive material which, upon solidification, visibly adhere to the microgranules. After a series of such exposures have been made, the accepting tape and donor web are separated, transferring microgranules to the accepting tape only in irradiated areas to provide the graphics. The graphics can be used on the accepting tape, or if the microgranules comprise a thermoplastic resin which acts as an adhesive upon softening, the graphics carried by the accepting tape can be adhered to a substrate simply by application of sufficient heat through the accepting tape to soften the thermoplastic resin. Moreover, the microgranules remaining on the donor web after separation of the accepting tape can be adhered to a substrate by the application of heat through the donor web, and the donor web may be peeled away to leave a negative of the graphics.
While that system can form graphics with such resolution that even half-tone photographs may be reproduced, the device required to expose the strip material (described in U.S. Pat. No. 3,828,358) is more complex and expensive than might be desired. The adhesive and microgranular layers will not adhere to each other unless they are exposed to intense radiation while the layers are in intimate contact. Such intimate contact must be achieved by complex mechanisms in the exposure device which press the layers together under great pressure in such a manner so as to remove air from between the layers.